The present invention relates to a product having reduced friction and improved abrasion resistance made from fluoroplastics used for a nonlubricated bearing, a dynamic seal and the like, and particularly to a product having reduced friction and improved abrasion resistance having excellent abrasion resistance.
Fluoroplastics are excellent in electric characteristics as well as in resistance to chemicals and heat, so that they have been extensively utilized for a variety of applications for both an industrial and a consumer use. However, such fluoroplastics may not be used in a sliding environment due to remarkable abrasion. In this respect, it has been tried to add a certain filler to fluoroplastics for the sake of improving abrasion resistance or deformation of a molded material of fluoroplastics.
Such filler, however, impairs excellent characteristics inherent to the fluoroplastics. As a result, it has had limited uses according to circumstances and has not always been satisfactory.
Accordingly, an object of the present invention is to provide a fluoroplastic product having reduced friction and improved abrasion resistance having excellent abrasion resistance and which maintains its excellent characteristics inherent thereto.
According to the feature of the present invention, a product having reduced friction and improved abrasion resistance comprises:
a fluoroplastic molded material containing a modified fluoroplastic prepared by applying ionizing radiation to a fluoroplastic (I) heated at the melting temperature or a higher temperature under an inert gas atmosphere having 10 torr or less oxygen concentration within a range of irradiation doses of from 1 KGy to 10 MGy; and an unmodified fluoroplastic (II).
According to an embodiment of the invention, it is preferred that a ratio of incorporation of the modified fluoroplastic and the unmodified fluoroplastic ranges from 10 to 90% by weight of the former with respect to from 90 to 10% by weight of the latter. In this respect, there is such a tendency that the larger amount of the latter results in the lower coefficient of abrasion, so that its abrasion wear increases.
An example of the fluoroplastics (I) and (II) used for the present invention includes tetrafluoloethylene-based polymers (hereinafter referred to as xe2x80x9cPTFExe2x80x9d), tetrafluoroethylene-perfluoro(alkyl vinyl ether)-based copolymers (hereinafter referred to as xe2x80x9cPFAxe2x80x9d), and tetrafluoroethylene-hexafluoropropylene-based copolymers (hereinafter referred to as xe2x80x9cFEPxe2x80x9d).
The above described PTFE includes also those containing 1 mol % or less of a polymeric unit derived from a copolymerizable monomer such as perfluoro(alkyl vinyl ether), hexafluoropropylene, (perfluoroalkyl)ethylene, and chlorotrifluoroethylene. Furthermore, the fluoroplastics in the above described copolymeric form may contain a small amount of a third component in their molecular structures.
In the present invention, it is preferred that the heat of crystallization in a fluoroplastic before irradiation is 50 J/g or less. When the quality exceeds such value as described above, the abrasion resistance of the fluoroplastic decreases. The heat of crystallization in the present invention is determined as follows: For measuring the heat of crystallization, the temperature rise and temperature descent of the fluoroplastic are repeated two times, respectively, per 10xc2x0 C./min. within a range of from 50 to 360xc2x0 C. The heat of crystallization of the fluoroplastic is calculated from the peak area, which is an area under a corresponding peak. Herein, the peak area is defined by a base line of a DSC (Differential Scanning Calorimeter) curve and one peak of the DSC curve in case of the second temperature descent.
Applications for a product having reduced friction and improved abrasion resistance according to the present invention include, for example, a nonlubricated bearing, a dynamic seal, rolls for copying machine, and a bearing pad. In this respect, it is expected to use a product having reduced friction and improved abrasion resistance according to the present invention in a field wherein fluoroplastics have heretofore been applied hardly.
As a specific manner for preparing a product having reduced friction and improved abrasion resistance of the present invention includes a method for pressure molding a mixture of a modified powder of fluoroplastics which has been exposed to ionizing radiation and a powder (or pellets) of fluoroplastics which has not yet been exposed to ionizing radiation, and a method for admixing a fluoroplastic with another heat-resisting plastic material, and then molding the resulting admixture into a form of product having reduced friction and improved abrasion resistance.
In the present invention, an example of ionizing radiation to be used includes xcex3 rays, electron rays, X rays, neutron radiation and high-energy ions. It is preferred to expose fluoroplastics to ionizing radiation in the absence of oxygen. Furthermore, it is preferred that an irradiation dose of ionizing radiation is within a range of from 1 KGy to 10 MGy. A more preferable irradiation dose is within a range of from 10 KGy to 1500 KGy in view of improvements in characteristics of low friction, abrasion resistance, and resistance to load of fluoroplastics.
In case of applying ionizing radiation, it is necessary for heating a fluoroplastic used at its crystalline melting point or a higher temperature. More specifically, when a PTFE is used as a fluoroplastic material, it is preferred to expose to ionizing radiation the fluoroplastic material which is under a heating condition at a higher temperature than 327xc2x0 C. being the crystalline melting point of the PTFE used. In case of employing a PFA or an FEP, it is required to expose such a material to ionizing radiation under a heating condition wherein the former PFA is heated at its crystalline melting point of 310xc2x0 C. or a higher temperature, while the latter FEP is heated at its melting point of 275xc2x0 C. or a higher temperature.
To heat a fluoroplastic at its crystalline melting point or a higher temperature means to activate molecular motion of backbone chains which constitute the fluoroplastic, whereby it becomes possible to efficiently accelerate crosslinking reactions among molecules. However, excessive heating brings about adversely cutting and decomposition of the molecular backbone chains. Therefore, a heating temperature should be limited to a range wherein it is 10 to 30xc2x0 C. higher than a crystalline melting point of fluoroplastics in view of suppressing an occurrence of such a depolymerizing phenomenon.